Interpretations:Canada Haunts Me

"Black horses" could refer to the dark horse candidate, Polk.
Or it could actually be talking about horses.

And "fifty-four forty or fight" was Polk's campaign slogan to aquire the Oregon territory. 54-40 was the northern parallel of the territory, with Russia's Alaska to the north. Oregon was owned in a joint sovereignty between the US and Britain at this time, and Polk was ready to control the land if he won the presidency.

Thinking back to Human Resources, The Skullivan remembers where he and The Spine were suggested to go: Canada. Both The Skullivan and The Spine show unreasonable fear of Canada, indicating to us that they may have mental issues, and that the story doesn't take place in Canada, in case you were wondering. Instead, they resolve to go to The Other Side Of The World.